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minor planet,

small body orbiting the sun. More than 300,000 asteroids have been identified and cataloged; more than a million are believed to exist in the main belt between Mars and Jupiter, with many more in the Kuiper belt beyond Neptune. Most asteroids are irregularly shaped, unlike the spherically shaped major planets. The largest asteroids, Quaoar (diameter c.800 mi/1,300 km) and Ixion and Varuna (both c.750 mi/1,200 km), reside in the Kuiper belt (see cometcomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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) and are about half the size of Pluto. The largest main-belt asteroid, CeresCeres
, in astronomy, a dwarf planet, the first asteroid to be discovered. It was found on Jan. 1, 1801, by G. Piazzi. He took three distinct observations; on the basis of these the mathematician Gauss calculated Ceres' orbit with such accuracy that it was found one year later
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, has a diameter of c.630 mi (1,000 km) and is considered a dwarf planetdwarf planet,
a nonluminous body of rock or gas that orbits the sun and has a rounded shape due to its gravity. Unlike a planet, a dwarf planet is not capable of clearing its orbit of smaller objects by collision, capture, or other means.
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; the three next largest in size are PallasPallas
, in astronomy, 2d asteroid to be discovered. It was found in 1802 by H. Olbers. The second largest asteroid, it has a diameter of c.300 mi (480 km). Its orbit has a semimajor axis of 2.78 astronomical units and a period of 1,684 days.
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, VestaVesta
, in astronomy, the fourth asteroid to be discovered. It was found in 1807 by H. Olbers. It is the third largest asteroid in size, with a diameter of c.326 mi (525 km). Its average distance from the sun is 2.36 astronomical units, and the period of its orbit is 1,325 days.
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, and JunoJuno
, in astronomy, 3d asteroid to be discovered. It was found in 1804 by C. Harding. It has a diameter of c.120 mi (190 km). Its average distance from the sun is 2.67 astronomical units, and its orbital period is 1,594 days.
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. Only Vesta can be seen with the naked eye. Many known asteroids are no larger than a few kilometers; in 1991, an asteroid only 33 ft (10 m) in diameter was found. Many asteroids are so small that their sizes cannot be measured directly; in many cases, their sizes have been estimated from their brightness and distances. The average orbital distance of the main-belt asteroids from the sun is about 2.9 astronomical unitsastronomical unit
(AU), mean distance between the earth and sun; one AU is c.92,960,000 mi (149,604,970 km). The astronomical unit is the principal unit of measurement within the solar system, e.g., Mercury is just over 1-3 AU and Pluto is about 39 AU from the sun.
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The orbitsorbit,
in astronomy, path in space described by a body revolving about a second body where the motion of the orbiting bodies is dominated by their mutual gravitational attraction.
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 of most main-belt asteroids lie partially between the orbits of Mars and Jupiter. These asteroids, also known as cisjovian asteroids, are divided into subgroups named for the main asteroid in the grouping: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles, and Hildas. The near-earth asteroids, which closely approach the earth, are classed as Atens (with orbits between the earth and the sun), Apollos (with orbits similar to that of the earth), and Amors (with orbits between the earth and Mars). Asteroids that share a planet's orbit are known as Trojan asteroidsTrojan asteroids,
asteroids that revolve about the sun in the same orbit as a planet, occupying stable positions (known as Lagrangian points) either about 60° ahead of the planet in the orbit or about 60° behind it.
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. First found in Jupiter's orbit, Trojan asteroids have also been found in the orbits of Mars, Neptune, and earth. Centaurs are asteroids with orbits in the outer solar system.

Asteroids are also classified by composition and albedoalbedo
, reflectivity of the surface of a planet, moon, asteroid, or other celestial body that does not shine by its own light. Albedo is measured as the fraction of incident light that the surface reflects back in all directions.
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, most being one of three types. The majority (C-type) are similar to carbon-chrondite meteorites with approximately the same composition as the sun (excluding hydrogen) and are relatively dark. Those with a composition of nickel iron mixed with silicates of iron and magnesium (S-type) are relatively bright. The M-type are composed of nickel iron and are bright. Some of the Trojan asteroids appear to be captured comets, composed of ice and dirt, rather than rocky asteroids.

Toward the end of the 18th cent. astronomers were searching for a planet whose orbit should, according to Bode's lawBode's law
[for J. E. Bode], also known as Titius's law or the Titius-Bode law, empirical relationship between the mean distances of the planets from the sun. If each number in the series 0, 3, 6, 12, 24, … (where a new number is twice the previous number) is increased by
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, have an average distance from the sun of 2.8 AU On Jan. 1, 1801, G. PiazziPiazzi, Giuseppe
, 1746–1826, Italian astronomer, a Theatine priest from 1769. He became (1781) professor of mathematics at the Univ. of Palermo, supervised construction of a government observatory (opened 1791) at Palermo, and was its first director.
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 discovered Ceres while studying the sky in the constellation Taurus; Ceres was later found to have an orbit very near that predicted by Bode's law. Ceres and the asteroids Juno, Pallas, and Vesta, which were discovered soon (1802–7) after Ceres, were initially regarded as planets by many astronomers, a view that was not overturned until additional asteroids were identified in the 1840s and 50s. By 1890 more than 300 asteroids had been discovered by visual means. In 1891, Max Wolf introduced the method of identifying an asteroid by the record of its path on an exposed photographic plate; it appears as a short line in a time exposure, rather than as the sharp point of a star. Brucia was the first asteroid discovered by this method. A more modern approach uses two photographs taken less than an hour apart and examined through a stereomicroscope that allows the asteroid to appear suspended above the background of stars. Still more modern techniques were employed in the discoveries of Ixion, found in 2001 using virtual telescopevirtual telescope,
a computerized interferometer (see interference) that merges the images from two or more telescopes to obtain a single, large, enhanced image. The image in each telescope is made from electromagnetic waves (light waves from an optical telescope, radio waves
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 techniques, and of Quaoar, found in 2002 using photographs taken with the Hubble Space TelescopeHubble Space Telescope
(HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km)
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Roughly 1,500 asteroids 330 ft (100 m) or larger in diameter have been identified that regularly intersect the orbit of the earth at a distance of 0.5 AU or less (half the distance to the sun). Over geologic time, similarly sized asteroids in similar orbits have struck the earth, and it is estimated that some 4,700 of these asteroids exist. NASA has estimated that a so-called potentially hazardous asteroid (or comet) might cause a global disaster once in several hundred thousand years, and a local disaster once every 10,000 years. Hermes, discovered in 1937 and subsequently lost until 2003 when it was identified as a pair of asteroids, comes within 378,000 mi (608,000 km), and Eros comes within 14 million mi (22 million km). More recently, a small asteroid provisionally designated 2002 MN, 150–360 ft (45–110 m) in diameter, passed within 75,000 mi (121,000 km) of the earth—about a third of the distance to the moon—in 2002. In 2009 an apparent asteroid impact was observed on Jupiter; estimated to be 0.3 mi (.5 km) wide, the object would have caused widespread devastation had it struck the earth.

Even smaller asteroids, between 100 ft and 330 ft (30 m and 100 m) in diameter, are capable of causing significant damage should they strike near a population center, and astronomers have observed about several hundred small asteroids, most measuring less than 55 yd (50 m) across, in near-earth orbits that are spread thinly between the earth and Mars. Many of these small asteroids have orbits that intersect the earth's. In 2013 one such asteroid, 2012 DA14, 150 ft (45 m) in diameter, passed by the earth only 17,200 mi (27,700 km) away. A stony asteroid of roughly similar size may have been responsible the widepsread destruction that occurred in 1908 in the isolated Tunguska BasinTunguska Basin,
c.400,000 sq mi (1,036,000 sq km), Krasnoyarsk Territory and Sakha Republic, E central Siberian Russia, between the Yenisei and Lena rivers. It has a huge untapped coal reserve.
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. A study of data collected by the Comprehensive Nuclear Test Ban Treaty Organization from 2000 to 2013, in which 26 asteroid explosions, largely in the atmosphere and over the oceans, were recorded, has led researchers associated with the B612 Foundation to conclude that an asteroid capable of destroying a city strikes the earth about once every 100 years.

Asteroids have been implicated in several mass extinctionsmass extinction,
the extinction of a large percentage of the earth's species, opening ecological niches for other species to fill. There have been at least ten such events.
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 of large numbers of animal and plant species in the past. From evidence found in sediments, Luis Walter AlvarezAlvarez, Luis Walter,
1911–88, American physicist, b. San Francisco, grad. Univ. of Chicago, 1932, Ph.D. 1936. He was awarded the 1968 Nobel Prize in Physics for his discovery of a large number of residence states (subatomic particles that have very short lifetimes and
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 and others hypothesized that the great mass extinction of the dinosaurs and other species 65 million years ago, at the end of the Cretaceous periodCretaceous period
, third and last period of the Mesozoic era of geologic time (see Geologic Timescale, table), lasting from approximately 144 to 65 million years ago. The Cretaceous was marked, in both North America and Europe, by extensive submergences of the continents.
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, was caused by the atmospheric and climatic effects of an asteroid impact; a possible crater exists in the Yucatán region of Mexico. In 1992, scientists reported that the appearance of patterns of shattered quartz crystals imbedded in Triassic shale and other fossil evidence suggest that another major mass extinction, about 200 million years ago, was caused by three closely spaced asteroid impacts.

The origin of asteroids is unclear; one theory claims that they were formed from material that could not condense into a single planet because of perturbation effects involving Jupiter. Some asteroids are actually nuclei of cometscomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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 that are no longer active.

The space probe Galileo, which passed near and photographed Gaspra (1991) and Ida (1993), provided the first close images of an asteroid. The pictures revealed that Ida has a natural satellite, Dactyl. Ida, in the main asteroid belt between Mars and Jupiter, is about 35 mi (56 km) long and 15 mi (24 km) in diameter. Its tiny moon is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida. Since then several other asteroids have been found to have companions, leading astronomers to believe that it may not be uncommon. The probe NEAR-Shoemaker examined Mathilde (1997) on its way to rendezvous (1999) and orbit (2000) Eros. After providing the most information ever obtained about an asteroid (measurements of size, shape, mass, and gravitational field; elemental and mineral composition of the surface; topographic mapping; and measurement of the magnetic field and its interaction with the solar wind), NEAR-Shoemaker made an originally unplanned landing on Eros in 2001, returning close-up images as it descended and data about surface composition. In 1999 the probe Deep Space 1 accomplished the then closest-ever flyby of an asteroid, coming within 16 mi (26 km) of the surface of BrailleBraille
, in astronomy, a small asteroid notable because it has the same atypical geologic composition as the larger asteroid Vesta. In 1999 the space probe Deep Space 1 passed within 16 mi (26 km) of Braille's surface, the closest flyby ever of an asteroid.
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; spectroscopic data suggests that Braille broke off from Vesta millions of years ago. Other probes have since studied asteroids, including Rosetta, Hayabusa, which returned with samples of the asteroid Itokawa in 2010, and Dawn, which studied Vesta in 2011–12 and then orbited Ceres beginning in 2015.


See T. Gehrels and M. S. Matthews, ed., Hazards Due to Comets and Asteroids (1995); J. S. Lewis, Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment (1997); C. T. Russell, ed., The Near Earth Asteroid Rendezvous Mission (1998); N. F. Michelson, The Asteroid Ephemeris 1900–2050 (1999).



(also called minor planet), one of numerous heavenly bodies revolving around the sun in elliptical orbits and distinguished from the nine planets by small dimensions. The largest asteroids are Ceres, Pallas, and Vesta, with diameters of 770,490, and 385 km, respectively. The other asteroids are much smaller. All the large asteroids are now known, and only asteroids with diameters less than 40 km have been discovered in the 20th century. Asteroids whose orbits have been precisely calculated are given a name and a permanent number. Thus, the asteroid discovered at the Simeiz Observatory in 1916 was named Vladilena in 1924 in honor of V. I. Lenin and was assigned the number 852. Information on all numbered asteroids is published annually in the special international publication Efemeridy malykh planet (Minor Planet Ephemerides), which has been issued since 1947 by the Institute of Theoretical Astronomy of the Academy of Sciences of the USSR in Leningrad. The 1974 edition contains the orbital elements of 1,796 asteroids and other data. About 40,000 asteroids can be observed using modern telescopes. Their total mass has been estimated to be one-thousandth that of the earth’s.

Asteroids revolve around the sun in the same direction as the major planets, but their eccentricities and the inclinations of their orbits on the average are significantly greater than those of the planets (the mean value of the eccentricity is 0.151, and the mean inclination to the plane of the ecliptic is 9.54°). The study of the motion of the asteriods is of great importance for solving a number of problems in astronomy, such as the calculation of the astronomical constants and the determination of the systematic errors in star catalogs. Analysis of perturbations in the motions of the asteroids also makes it possible to determine the masses of the principal planets.

Ceres, the first asteroid to be sighted, was discovered by chance on Jan. 1, 1801, by the Italian astronomer G. Piazzi in Palermo, Sicily. Piazzi attempted to calculate the orbit of the new celestial object but was unsuccessful and after 1½ months lost track of the object. Soon after, the German scientist K. Gauss developed a new method of computing orbits from three observed positions, which is still used. Using Gauss’ ephemerides, the asteroid was rediscovered by the German astronomer H. Olbers exactly one year after its initial detection. The orbital elements obtained by Gauss demonstrated that Ceres moved between the orbits of Mars and Jupiter and that its semimajor axis a was equal to 2.8 astronomical units (AU), which is very close to the value predicted by Bode’s law. In March 1802, Olbers discovered a second asteroid, Pallas, for which a also equaled 2.8 AU. In September 1804, the German astronomer C. Harding discovered a third asteroid, Juno (a = 2.7 AU), and in March 1807, Olbers discovered Vesta (a = 2.4 AU). The similarity of the numerical values of the semimajor axes of all four asteroids led Olbers to formulate the hypothesis that all of them are fragments of one planet that had disintegrated as a result of a cosmic catastrophe.

A fifth asteroid, Astraea (a = 2.6 AU), was discovered only in 1845. Since then, discoveries of new asteroids have continued without intermission. In 1898 an asteroid possessing an unusual type of motion was discovered. Its orbit intersected the orbit of Mars and approached that of the earth closer than the orbits of the major planets. This asteroid is now called Eros. (Asteroids are generally given feminine names; those with distinctive features are assigned masculine names.) At its closest approach to the earth, the distance of Eros is 22,500,000 km, whereas the distance of Mars is never less than 56,000,000 km, and that of Venus, never less than 40,000,000 km. A whole group of asteroids with similar orbits has since been discovered, including Hermes (1937), which passed within 700,000 km of the earth.

Yet another impressive astronomical discovery was made in 1907 when the asteroid Achilles, whose orbit is very close to that of Jupiter, was discovered. Achilles completes periodic motions about the libration points. In 1920 the asteroid Hidalgo was discovered, whose orbit stretches between the orbits of Mars and Saturn. Hidalgo travels further from the sun (a = 5.8 AU) than any other asteroid. It moves in a plane inclined to the plane of the ecliptic at an angle of 42.5°. The aphelion of Hidalgo is approximately equal to the mean distance of Saturn to the sun.

The asteroid Icarus, discovered in 1949, is of great interest. Its orbit is similar to that of a short-period comet with a semimajor axis equal to 1.08 AU. At perihelion, Icarus travels deep within the orbit of Mercury and passes within 0.19 AU (28,000,000 km) of the sun. Icarus periodically passes very close to the earth (about 6,000,000 to 7,000,000 km) and to Mercury (up to 12,000,000 km). It was the first asteroid to be observed by radar (1968). These observations made it possible to accurately calculate the distance to the asteroid as it passed the earth, as well as the asteroid’s dimensions (diameter about 1 km) and axial rotation (about 2 hr). Icarus’ axial rotation proved to be the smallest of all the asteroids. Although the images of Icarus and Hidalgo lack any gaseous envelope, we may assume that both of them are cometary nuclei that have already exhausted their gas reserves.

Photometric observations of the asteroids are of great importance and provide data on the physical nature of heavenly bodies. Determination of the brightness and the study of the variation of the brightness make it possible to determine the asteroid’s axial rotation, approximate dimensions, and shape. Such studies have shown that the asteroids are of irregular shape and that their periods of rotation range from two to 17 hr. Direct micro-metric measurements of angular diameters are possible only for some of the larger asteroids. Characteristically, the periods of rotation of the asteroids are on the same order of magnitude as those of the giant planets Jupiter, Saturn, Uranus, and Neptune.

The asteroids apparently formed in the process of the successive fragmentation caused by collisions among a few (10-100) larger parent bodies that had formed during the evolution of protoplanetary matter at the same time as the principal planets. The probability of such collisions is relatively large in the cotemporary epoch owing to the large number of asteroids. Such collisions must occur quite often. Meteoroids, cosmic dust, and new asteroids are the products of collisions. Spontaneous destruction of irregularly shaped asteroids is also possible. Their periodic heating and cooling as well as the effect of tidal forces from the major planets disrupts the internal structure of the asteroids, and if the velocity of their rotation is close to the critical velocity, the asteroids may disintegrate.

The great volume of observations and computational work associated with the study of asteroids has resulted in the organization of a special international service. Since 1911 the processing of observations and the calculation of orbits have been chiefly concentrated at the Berlin Astronomical Institute. Other scientific groups also actively participate there. Calculation of the orbits and ephemerides of the asteroids has been carried out since 1946 by the Institute of Theoretical Astronomy of the Academy of Sciences of the USSR at the request of the International Astronomical Union. A scientific center for the observation of asteroids was established in 1948 at the astronomical observatory in Cincinnati, Ohio.

Systematic observations of the asteroids in the USSR are carried out at the Crimean Astrophysical Observatory and at other observatories.


Putilin, I. I. Malye planety. Moscow, 1953.
Zigel’, F. Iu. Malye planety. Moscow, 1969.
Iakhontova, N. S. “Malye planety.” Astronomicheskii vestnik, 1969, vol. 3, no. 4.
Malye planety. Edited by N. S. Samoilova-Iakhontova. Moscow, 1973.



One of the many small celestial bodies revolving around the sun, most of the orbits being between those of Mars and Jupiter. Also known as minor planet; planetoid.


1. any of numerous small celestial bodies that move around the sun mainly between the orbits of Mars and Jupiter. Their diameters range from 930 kilometres (Ceres) to less than one kilometre
2. any echinoderm of the class Asteroidea; a starfish
References in periodicals archive ?
He suggests the jump was caused by the force of the asteriod, which, according to the impact theory, was large enough to shoot vast clouds of dust and debris into the air, blocking light to the entire planet and causing or contributing to the demise of about three-fourths of the world's animal species, including the dinosaurs.
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